Introduction

A rod-like molecule of carotenes lacks polar groups and therefore may be expected to be localized in the hydrophobic core of the lipid membrane, owing to the requirement of energy minimization in a system. On the other hand, the terminal groups of polar carotenoids are expected to interact with the polar head group regions of a lipid bilayer via hydrogen bonds. Such a localization of carotenoids, deduced on the basis of their chemical structure, has a strong experimental support from the analysis of a position of light absorption maxima of carotenoid pigments incorporated to lipid membranes (1-4). Specifically, the positions of absorption maxima of carotenoid pigments incorporated to lipid membranes correlate with the polarizability term of the hydrophobic core of the lipid bilayer, representing dielectric properties of the chromophore environment and calculated on the basis of a refractive index. Figure 1 presents such a dependency plotted for lutein, dissolved in a series of organic solvents and incorporated into liposomes formed with dipalmitoylphosphatidylcholine (DPPC). The value of the polarizability term for the hydrophobic core of DPPC correlates very well with the position of the 0-0 transition in the absorption band of lutein incorporated into this membrane system. The rule that polar end groups of xanthophyll pigments have to remain in direct contact with polar groups of lipid molecules, realized in most cases by hydrogen bonding, determines the orientation of carotenoid molecules with respect to the lipid bilayer, as will be discussed below. Both localization and orientation of carotenoid molecules in the membrane are directly responsible for molecular

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Figure 1 Dependence of the position of the 0-0 vibrational transition in the main electronic absorption band of lutein dissolved in several organic solvents of the refractive index n on the polarizability term. The position of the absorption maximum of lutein embedded to DPPC liposomes [20-879 cm"1 (5)] indicated with the dashed line and the value of a polarizability term for the hydrophobic core of DPPC membrane in the La phase [0.2424 (6)] indicated by the arrow.

mechanisms of carotenoid-lipid interaction that influence basic physical properties of lipid membranes such as membrane thickness, fluidity, permeability, energy, and cooperativity of phase transitions, etc. Selected aspects of physiologically relevant carotenoid-lipid interactions, directly dependent on carotenoid orientation with respect to the lipid bilayer will also be addressed.

II. ORIENTATION OF CAROTENOIDS IN LIPID MEMBRANES

Figure 2 presents main different patterns of orientation of carotenoid pigments in lipid membranes: not well-defined orientation (as in the case of nonpolar ^-carotene), roughly vertical (as in the case of polar zeaxanthin), horizontal (as in the case of cis-zeaxanthin), or both horizontal and vertical (as in the case of lutein).

Figure 2 Model of localization and orientation of different in structure carotenoid pigments in the hydrophobic core of lipid membrane. See text for discussion.

Polar groups of xanthophylls are bound to the terminal rings, in almost all physiologically relevant pigments, such as zeaxanthin, lutein, violaxanthin, and astaxanthin. Such localization of polar groups allows the prediction of two essentially different orientation patterns of polar carotenoid pigments in the lipid bilayer: vertical (Fig. 3) and horizontal with respect to the plane of the membrane. The pigment system of C=C bonds has to be located in the hydrophobic core of the membrane in all cases, but polar groups will be anchored in the same head group region or in the opposite polar zones of the bilayer, in the case of horizontal and vertical pigment orientation, respectively. For stereochemical reasons, not all terminally bound polar groups of xanthophylls can remain in contact with the same hydrophobic-hydrophilic interface of the membrane simultaneously. This means that horizontal orientation will be limited to a selected number of polar carotenoids, such as lutein. Lutein and zeaxanthin, the macular pigments, are identical in their chemical composition and very close in structure. Despite that, one essential difference appears that may be responsible for different localization and orientation of these two xanthophyll pigments within a lipid bilayer. Lutein and zeaxanthin contain 11 double bonds but one double bond, in the case of lutein (C4-C5), is not conjugated to the conjugated double-bond system, in contrast

Lutein Dipole Transition Moment

Figure 3 Orientation of the molecular axes of lutein (A) and the model of vertical orientation of the pigment incorporated to the lipid membrane (B). d is the thickness of the hydrophobic core of the membrane, dotted line the axis connecting opposite hydroxyl groups, dashed line the direction of the chromophore, continuous line the axis of the transition dipole moment tilted by about 15° with respect to the linear polyene chromophore (12).

Figure 3 Orientation of the molecular axes of lutein (A) and the model of vertical orientation of the pigment incorporated to the lipid membrane (B). d is the thickness of the hydrophobic core of the membrane, dotted line the axis connecting opposite hydroxyl groups, dashed line the direction of the chromophore, continuous line the axis of the transition dipole moment tilted by about 15° with respect to the linear polyene chromophore (12).

to the terminal double-bond of zeaxanthin (C5-C6). Such a difference, as may be judged from the spectroscopic point of view, seems to influence stereochemical properties of lutein considerably. Namely, a relative rotational freedom of the entire terminal ring of lutein around the C6-C7 bond (s ring) can be predicted, which is unlike in the case of the terminal ring of zeaxanthin. This particular property of lutein is most probably directly responsible for the differences in orientation of lutein and zeaxanthin in model lipid membranes, as determined by means of linear dichroism measurements, carried out in oriented lipid multibilayers composed of several lipid constituents (7,8). Orientation of xanthophyll pigments with terminally located polar groups, such as zeaxanthin (two hydroxyl groups at the C3 and C3 positions) can be predicted on the basis of information about the distance between polar groups of the pigment relative to the distance between the opposite polar zones of the lipid bilayer (thickness of the hydrophobic core of the membrane). In several lipid bilayers, the thickness of the hydrophobic core of the membrane [d = 2.26 nm in the case of egg yolk phosphatidylcholine (EYPC) and d = 2.54 nm in the case of dimyristoylpho-sphatidylcholine (DMPC)] (7) is less than the distance of hydroxyl groups of C40 xanthophyll pigments such as zeaxanthin [d = 3.2 nm, (1)] and therefore a tilted orientation of the pigment can be predicted. Such a prediction is in good agreement with experimental linear dichroism data, as can be seen from Table 1. Roughly vertical orientation of the axis connecting the polar groups located at the ends of xanthophyll molecules (Fig. 3) can be predicted in the case of membranes

Table 1 Orientation of Chromophore of Selected Carotenoid Pigments with Respect to the Axis Normal to the Plane of the Lipid Membrane Determined on the Basis of Linear Dichroism Measurements

Orientration

Carotenoid

Lipid component

angle (°)

Ref.

ß-Carotene

EYPC

55

7

DOPC

~90

9

1-Oleoyl-sn-glycerol

~90

10

DMPC

0 and 90

9

Zeaxanthin

EYPC

33

8

DMPC

25

2

DPPC

36

8

DGDG

9

11

MGDG

17

11

Lutein

EYPC

67

8

DPPC

57

8

DHPC

47

8

Violaxanthin

DMPC

22

2

DGDG

28

11

MGDG

35

11

Lycopene

EYPC

74

7

Astaxanthin

EYPC

26

7

ß-Cryptoxanthin

EYPC

38

7

EYPC, egg yolk phosphatidylcholine; DOPC, dioleoylphosphatidylcholine; DPPC, dipalmitoylphos-phatidylcholine; DMPC, dimyristoylphosphatidylcholine; DHPC, dihexadecylphosphatidylcholine; MGDG, monogalactosyldiacylglycerol; DGDG, digalactosyldiacylglycerol.

EYPC, egg yolk phosphatidylcholine; DOPC, dioleoylphosphatidylcholine; DPPC, dipalmitoylphos-phatidylcholine; DMPC, dimyristoylphosphatidylcholine; DHPC, dihexadecylphosphatidylcholine; MGDG, monogalactosyldiacylglycerol; DGDG, digalactosyldiacylglycerol.

characterized by a thickness of the hydrophobic core that matches the distance between the polar groups. Such a situation may be expected in the case of thylakoid membranes of chloroplasts (d = 3 nm) or the membranes formed with DPPC (d = 3.2 nm) (7). The much larger orientation angle values determined in the case of lutein than in the case of zeaxanthin (7,8) may be explained in terms of two pools of the pigment, the first one oriented parallel with respect to the plane of the membrane and the other one oriented the same way or close to that of zeaxanthin. The orientation angles for lutein, with respect to the axis normal to the plane of the membrane, determined as 67° in the case of EYPC or 57° in the case of DPPC correspond to horizontal and roughly vertical pools as 78% and 22% in the case of EYPC or 45% and 55% in the case of DPPC, respectively (8). No distinctly different orientations of lutein and zeaxanthin have been determined in the membranes formed with dihexadecylphosphatidylcholine (DHPC), 47° and 37°, respectively (with the experimental error 4-5°) (8). The fact that DHPC is an analog of DPPC without the keto groups located at the polar-nonpolar interface of the lipid bilayer indicates that the terminal hydroxyl groups of lutein molecules oriented in the plane of the membrane are localized at the interface and interact most probably with the keto groups of lipids.

An essentially different situation, in terms of structural determinants of carotenoid orientation in lipid bilayers, can be expected for membranes containing carotenoid pigments lacking any polar groups that may determine specific pigment localization and orientation. The van der Waals forces between pigment chromophores and alkyl chains of lipid molecules seem to be the sole type of interaction that can potentially influence pigment orientation. Such a situation takes place in membranes containing ^-carotene or lycopene. The orientation of lycopene with respect to the axis normal to the plane of the membrane determined as 74° and the orientation of ^-carotene determined as 55° in the same system of EYPC membranes (Table 1) are larger or exceptionally close to the magic angle (54.7°), respectively. These results can be interpreted as an indication of roughly horizontal or not well-defined orientations of lycopene and ^-carotene, respectively, most probably within the central part of the hydrophobic core of the membrane (Fig. 2). Parallel orientation of ^-carotene has also been determined in other experimental systems (Table 1). In some cases, a second possibility of orientation, defined as orthogonal to the parallel one, has been additionally identified for ^-carotene (9). This provides a further indication of a complex organization of lipid membranes containing nonpolar carotenoids.

The discussion above refers to the all-trans carotenoid pigments, particularly relevant from the physiological point of view. On the other hand, carotenoid pigments in cis conformation are also lipid membrane located and are expected to influence membrane properties at least at a degree comparable to the trans stereoisomers, as can be deduced from the monomolecular layer studies of two-component pigment-lipid systems (Milanowska and Gruszecki, unpublished work). Monomolecular layer technique studies reveals that 9-cis-and 13-cis-zeaxanthin is oriented in such a way that both hydroxyl groups face the polar-nonpolar interface in the environment of DPPC, at the surface pressure values characteristic of natural biomembranes.

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A Disquistion On The Evils Of Using Tobacco

A Disquistion On The Evils Of Using Tobacco

Among the evils which a vitiated appetite has fastened upon mankind, those that arise from the use of Tobacco hold a prominent place, and call loudly for reform. We pity the poor Chinese, who stupifies body and mind with opium, and the wretched Hindoo, who is under a similar slavery to his favorite plant, the Betel but we present the humiliating spectacle of an enlightened and christian nation, wasting annually more than twenty-five millions of dollars, and destroying the health and the lives of thousands, by a practice not at all less degrading than that of the Chinese or Hindoo.

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